Piercings aren't just done to ears. People have their nipples, navels, eyebrows, clitoris hoods, penises, labia, and other body parts pierced, and bits of metal permanently in the piercings. These are the bare facts, but this practice, it turns out, has a lot to do with S&M.
Getting a piercing, first of all, is an incredibly intense rush, on a purely physical level. It's a very powerful thing to willingly have someone push a sharp piece of metal through your body. It can be a level of sensation beyond any you've experienced in your life.
Once you have the piercing, it can completely change the way that part of the body feels to you. One friend of mine said that his nipple piercings turned his nipples from little places that felt OK to full-fledged erotic zones connected right to his cock. He calls his nipple piercings the best thing he's ever done for his body and his sex life, and he seems damn sincere! The same goes for all the piercings in the genital area; they can really make sex more fun.
There is some medical evidence that nerves around the pierced location become much more sensitive, so this isn't mere folklore. In case it wasn't clear, once a piercing heals, it doesn't hurt at all; quite the opposite!
Some people get into play piercings, which are done temporarily with very thin needles, which are removed at the end of the scene. This is basically another kind of sensory trip, which some find very enjoyable. The needles don't _hurt_, exactly, but you certainly do know they're in there, and they sure do get those endorphins pumping!
You don't want to try doing permanent piercings unless you've been personally trained by a professional; there is a lot of knowledge involved, and you definitely don't want to get stuck with a bad piercing. Play piercings are less hardcore, but you still want to make sure you know sterile technique (remember safe sex!).
Does nipple piercing cause problems with nursing? Sometimes yes, sometimes no; there are stories both ways. There are many milk ducts in an average nipple, so the chances are good that nursing can still happen, but nothing is certain.
For more information about piercings, see the newsgroup rec.arts.bodyart (or future updates of this section).
EDITOR'S NOTE: We will be adding more on this topic here soon but until then remember that some of the websites listed on the Sources page have much more information on specific types of piercings, equipment and techniques.